1. Field of the Invention
Aspects of the present invention relates to a thin film transistor (TFT), a method of fabricating the same, and an organic light emitting diode (OLED) display device having the same. More particularly, aspects of the present invention relate to a TFT having good electrical characteristics in which metal catalysts remaining in a channel region of a semiconductor layer crystallized using the metal catalysts are gettered to reduce the amount of the metal catalysts remaining in the semiconductor layer, a method of fabricating the same, and an OLED display device having the same.
2. Description of the Related Art
In general, a polycrystalline silicon layer can be advantageously applied to high field-effect mobility and high-speed operation circuits and adapted for complementary metal oxide semiconductor (CMOS) circuits. Polycrystalline silicon layers are widely used as semiconductor layers for TFTs. TFTs using a polycrystalline silicon layer are typically used as an active element of an active matrix liquid crystal display (AMLCD) and a switching element and a driving element of an OLED.
Methods of crystallizing amorphous silicon into polycrystalline silicon include a solid phase crystallization method, an excimer laser crystallization method, a metal induced crystallization method, and a metal induced lateral crystallization method. In the solid phase crystallization method, an amorphous silicon layer is annealed for several hours to tens of hours at a temperature of about 700° C. or less, which is the deformation temperature of glass that is used as the substrate of the display device in which the TFT is used. In the excimer laser crystallization method, local heating is carried out by irradiating an excimer laser onto an amorphous silicon layer for a very short time for crystallization. In the metal induced crystallization method, a phenomenon that the phase change from an amorphous silicon layer into a polycrystalline silicon layer is induced by metal such as nickel, palladium, aurum or aluminum by contacting the metal with the amorphous silicon layer or implanting the metal into the amorphous silicon layer is used. In the metal induced lateral crystallization method sequential crystallization of an amorphous silicon layer is induced while silicide generated by the reaction between metal and silicon continues to propagate laterally.
However, the solid phase crystallization method requires not only a long process time but a long annealing time at a high temperature so that the substrate is disadvantageously apt to be deformed. The excimer laser crystallization method requires a costly laser apparatus and causes protrusions on the polycrystallized surface, providing an inferior interface property between a semiconductor layer and a gate insulating layer. The metal induced crystallization method or the metal induced lateral crystallization method causes a large amount of metal catalysts to remain in the polycrystallized silicon layer so that the leakage current of the semiconductor layer of the TFT increases.
Currently, in the method of crystallizing an amorphous silicon layer using metal crystallization can be advantageously performed for a short time at a lower temperature than in the solid phase crystallization method. Therefore, much research has been conducted on metal induced crystallization methods. Crystallization method using metal include a metal induced crystallization (MIC) method, a metal induced lateral crystallization (MILC) method, and a super grain silicon (SGS) crystallization method. However, in methods using a metal catalyst, device characteristics of the TFT may be deteriorated because of contamination resulting from the metal catalyst.
Accordingly, in a crystallization method using a metal catalyst, a gettering process is performed after the amorphous silicon layer is crystallized in order to remove the metal catalysts. A general gettering process is performed using impurities such as phosphorous or noble gas or using a method of forming an amorphous silicon layer on a polycrystalline silicon layer. However, even in the methods as described above, the metal catalysts within the polycrystalline silicon layer may not be significantly removed so that the leakage current may remain high.